DRAFT for USWG Review Vulnerability Analysis and Resilient
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1 DRAFT for USWG Review Vulnerability Analysis and Resilient Design Considerations for Stormwater Best Management Practices Prepared by: David Wood, Chesapeake Stormwater Network January 2020 Photo Courtesy: District Department of Energy and Environment (DOEE)
DRAFT for USWG Review Vulnerability Analysis and Resilient
Considerations for Stormwater Best Management Practices
Prepared by: David Wood, Chesapeake Stormwater Network
January 2020
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Summary Climate change is expected to alter the volume and
distribution of precipitation across the Chesapeake
Bay watershed in the coming years. These changing hydrologic
conditions, especially when coupled with
ongoing development, pose a risk to stormwater infrastructure and
public safety. Declining performance
or outright failure of stormwater best management practices (BMPs)
adds further challenges to meeting
the water quality goals outlined by the Chesapeake Bay Total
Maximum Daily Load (TMDL).
This memo identifies the types of risk facing stormwater
infrastructure in the Chesapeake Bay
watershed, as well as the most vulnerable features of these
practices. Chesapeake Bay BMPs, including
both traditional stormwater practices and more non-traditional
restoration approaches, are covered.
The memo also identifies resilient stormwater design principles and
next steps that stormwater
managers and Chesapeake Bay Program stakeholders may pursue to
maintain and improve the long-
term performance of their BMPs. The following is a summary of the
key takeaways:
• Stormwater best management practices (BMPs) and conveyance
infrastructure face risks that
occur across a spectrum from catastrophic failure to diminishing
pollutant removal
performance. The risk type can be influenced by a variety of
different factors, including:
o The location of a practice in the urban landscape. Practices
higher in the watershed tend
to be more distributed, with smaller drainage areas than those
lower in the watershed.
In addition, practices that are “on-line”, or in the natural flow
of runoff, face greater
risks.
o Older practices were more likely to be designed using outdated
precipitation data and
design guidelines, and therefore more likely to be undersized for
future precipitation.
Older practices are also more likely to be experiencing natural
wear and tear, increasing
their vulnerability to extreme events.
o Design and maintenance condition. Many stormwater BMPs throughout
the watershed
are already at risk of failure due to a wide variety of design and
construction flaws, as
well as insufficient maintenance. Increasing storm intensity is
likely to exacerbate design
flaws, speeding up likely structural and performance failures,
while demanding more
routine maintenance to maintain water quality performance.
• Not all risks are driven solely by climate change – projected
development across the watershed
will continue to alter urban hydrology, while regular design and
maintenance challenges
continue to lead to practice failure even in the absence of direct
climate change impacts.
• While the specific vulnerabilities differ between the BMP types,
the most common
vulnerabilities include:
o More frequent overflow/bypass of runoff,
o Loss of treatment capacity due to sedimentation or high
groundwater tables,
o Increased erosion where runoff enters and exits the
practices.
• Overall, it is recommended that a Bay-wide effort be undertaken
to closely evaluate and update
stormwater design criteria and floodplain management regulations in
the context of projected
climate impacts and leading BMP vulnerabilities.
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2. Risk and the Urban BMP Landscape Page 4
3. Upland Low Impact Development (LID) Practices Page 7
4. The Conveyance Network Page 12
5. Ponds and Wetlands Page 15
6. Stream Corridors and Shorelines Page 18
7. Other Chesapeake Bay Restoration Approaches Page 21
8. Moving to Resilient Design Page 23
9. Conclusions and Proposed Next Steps Page 26
10. References Page 28
Background Maintaining and improving the resilience of stormwater
infrastructure and best management practices
(BMPs) is likely to be one of the most critical challenges faced by
Chesapeake Bay watershed
communities in the coming years. This is the fourth and final in a
series of memos produced for the
Chesapeake Bay Program partnership to clearly define the needs of
local stormwater managers and
identify the specific initiatives that will allow them to address
their restoration and public safety
functions under future climate conditions.
The first three memos outlined the greatest concerns of stormwater
managers with regards to the
resilience of their infrastructure and BMPs (Wood, 2020a), the
current stormwater engineering design
standards used across the watershed (Wood, 2020b), and the most
recent climate change projections
for the region, focusing on local precipitation intensity (Wood,
2020c). To best understand this memo, it
is important to first review some of the key findings from the
preceding work:
• Stormwater professionals across the watershed are not comfortable
with the current quality
and utility of engineering design criteria on future rainfall
intensity as currently provided by
state and/or federal authorities.
• Each state and the District of Columbia uses different design
criteria. Further, within states,
there are often differences in how design storms and precipitation
data sources are discussed
by departments of transportation, environmental regulatory agencies
and the departments
overseeing dam safety. With one or two exceptions, the most recent
wave of state
stormwater manual updates occurred between 2006-2013.
• Engineering design criteria and stormwater runoff models
generally rely on historic
precipitation data. The most commonly used dataset, Atlas 14, has a
period of record that is
already twenty years old, while multiple studies suggest that
stormwater design based on
historic precipitation analysis are likely to underestimate future
precipitation.
• Work to develop climate change projections with the temporal and
geographic resolution
needed for stormwater modeling show that precipitation intensity is
generally expected to
increase by 5-35% by the middle of the 21st century. Projections
also predict approximately 2-
3°F of warming and 1-2 feet of sea level rise in the same time
period.
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Each of these factors contribute to increasing risks and
vulnerabilities to both existing and future
stormwater infrastructure and BMPs. This memo is based on the idea
that climate change-induced risk
occurs on a spectrum that ranges from catastrophic failure to
diminished pollutant removal function. It
also considers failure and loss of function that may have occurred
even in the absence of climate
change. Where practices occur within the urban landscape, the age
of the practice, design flaws, and
current maintenance conditions all impact the type of risk.
Risk and the Urban BMP Landscape Stormwater BMP failure can take
different forms. Failure occurs on a spectrum with varying levels
of risk
based upon the type of BMP, its position in the urban landscape and
the ultimate management
objectives the practice is intended to address. Therefore, the
increasing volume and intensity of
precipitation projected for the Chesapeake Bay watershed in the
coming decades does not necessarily
mean the same type of impacts for all types of infrastructure, and
the decisions about how to build
resiliency will be linked to the primary management
objectives.
Figure 1. Climate Change-Induced Risk Spectrum for Stormwater Best
Management Practices
Risk of Failure:
It should be unsurprising that catastrophic failure – damage to
critical public safety infrastructure such
as bridges, dams, roads, and culverts due to extreme storm events –
is the greatest concern of
stormwater managers across the watershed (Wood 2020a). But failure
can look different depending on
a practice’s location and management objectives. In the case of
structural failures, the indicators may be
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the same as those for catastrophic failure, but the BMP or
infrastructure is located or designed in such a
way that the there is no risk to public safety.
Performance failure can be divided into two sub-categories: water
quality failure and water quantity
failure. Water quality performance failures are among the most
challenging to detect. The pollutant
reduction performance of stormwater BMPs is notoriously variable,
both across sites as well as across
storm events. Performance failure, or even just diminished
performance, may be caused by an inlet
obstruction preventing runoff from entering a BMP, or a large
influx of sediment clogging the filter bed
and preventing infiltration. Often, multiple factors combine to
cause performance failure, making it
difficult to pinpoint a single solution.
Water quantity failures can impact downstream properties,
conveyance systems and, in some cases,
floodplain boundaries. In other words, a practice that no longer
captures the volume of runoff it was
designed to capture can lead to an increase in runoff being
delivered downstream, resulting in potential
impacts to lower lying portions of the watershed.
It is also important to realize that many vulnerabilities to
stormwater BMPs are already present and are
only further exacerbated by climate change. It is expected that a
number of stormwater BMPs will suffer
a previously described failure in the next few years, regardless of
future climate change. Design flaws,
ongoing development of the watershed, lack of routine maintenance,
or improper BMP siting and
construction may all play a role the long-term performance of
stormwater BMPs. Non-climate related
changes to the urban landscape are ongoing and continue to exert
significant impacts on runoff volume
and pollutant loads. For example, Wang et al. (2016) found that
peak runoff and total suspended
sediment loads were more sensitive to the changes in impervious
surface and progressive urban
intensification than to climate changes.
These failures have already occurred across each of the Chesapeake
Bay watershed jurisdictions and the
challenge is only becoming more pronounced with the ever-increasing
implementation needed to
achieve the Chesapeake Bay TMDL goals. A detailed evaluation of
over 200 stormwater BMPs in Virginia
found that 46% of BMPs were in need of some form of maintenance,
and that treatment was ineffective
in over a third of the practices due to a variety of design flaws
(Hirschman et al., 2009). In recent years,
many states and MS4 communities have significantly improved their
inspection and maintenance
programs, developing better asset management systems and devoting
more resources to staff training.
However, the rate of BMP implementation across the watershed has
led to a tremendous burden on
verification and maintenance programs. As a result of this
increasing maintenance burden, over 4,300
BMPs implemented for the Chesapeake Bay TMDL across the agriculture
and urban sectors were
reported to the Chesapeake Bay Program as having failed a
verification inspection as of 2019, while
another 58,300 expired due to a lack of reported inspection date
(CBP, 2020). Between failure and
expiration, that represents 19% of all reported BMPs.
The Urban BMP Landscape
The location of the BMP within the urban stormwater drainage
network is also critical to consider when
assessing its vulnerability to climate change. The urban stormwater
drainage network can be thought of
in four zones: upland practices, conveyance practices, ponds and
wetlands, and stream corridor
practices (see Figure 2).
Figure 2. The Urban Stormwater Drainage Network
As runoff moves “downstream” through the urban drainage network, it
becomes more concentrated
and the practices that capture, infiltrate, detain, or convey that
runoff are designed to handle
increasingly larger volumes of water. Small, highly dispersed
upland BMPs typically aren’t designed to
capture large volumes of runoff, but are so numerous that expected
increases in routine maintenance
needs will pose a risk to pollutant removal performance. Meanwhile,
conveyance practices often run
beneath or parallel to roads and other transportation corridors.
Storm events that damage or
overwhelm the conveyance network are likely to have a direct and
immediate impact on public safety.
Larger ponds and stream corridors are often at the low-points of
the urban landscape and therefore the
eventual collection point for stormwater runoff. Despite the best
efforts of practices implemented in the
previous zones, a significant amount of runoff eventually makes its
way the stream corridor, subjecting
practices and infrastructure in the corridor to the highest flows
during intense storm events.
In addition to the different levels of risk carried by these four
zones due to their location in the
watershed, the BMPs or infrastructure in these zones also tend to
be of varying age. The following is a
“typical” timeline for the progression of stormwater infrastructure
in most Bay communities, although
the precise timeline for each era varies in each community due to
the specific years in which new
engineering criteria were adopted:
• 1960’s and before: Conveyance only (no detention storage)
• 1980’s and before: Detention pond era (quantity control, no
quality control)
• 1990’s -2010: Quality and quantity control
• 2010 to present: LID era (and in recent years, the stream
restoration era)
Communities can generally predict the era based on the average age
of urban development in a local
watershed. Thinking about it a different way, the age of a
development can be used as an initial
screening criterion when conducting flood risk assessments. Between
a lack of detention in the
community design, the increased maintenance needs of aging
infrastructure, and outdated design
criteria, these neighborhoods will likely be hotspots for inland
flooding. Communities will need to target
these areas by mapping the age of development and the condition and
design era of their historic
stormwater infrastructure, and implementing corresponding watershed
strategies to reduce flooding
risks.
The subsequent sections provide a review of the vulnerability of
practices in each zone to the different
categories of failure.
SUMMARY OF VULNERABILITIES
Upland LID practices are at high risk of climate change related
impacts.
Upland practices are most vulnerable to erosion at the inlet and
outlet, clogging of the filter bed due to increased sediment loads,
shifting vegetation communities, and increased
bypass/overflow.
There are limited data quantifying the loss in pollutant removal
performance due to climate change. While studies show little change
in the 90th percentile storm event, suggesting little impact on LID
practice performance, both monitoring and modeling studies have
found increasing volumes of untreated overflow under climate change
scenarios.
While overflow/bypass is one potential culprit, the mechanisms
behind observed declines in pollutant removal performance are not
well understood. Existing studies on changes to pollutant removal
performance are almost entirely based on modeled data and show
declining pollutant removal efficiencies on the order of
0-15%.
While climate change is expected to exacerbate design and
maintenance flaws, declining pollutant removal performance is
already being observed due to the age and poor maintenance
condition of many existing practices, as well as increasing
impervious cover across the urban and ex-urban landscape.
The BMPs:
• Manufactured Treatment Devices
Upland LID practices are designed to treat small and moderate storm
events. They are widely distributed
across the landscape, with each practice typically treating a
relatively small contributing drainage area.
The practices are not usually designed to withstand runoff volumes
or velocities above the 2-year storm
event. Thus, extreme storms pose a high risk of damage or failure
to individual practices, increasing the
need for corrective maintenance.
Vulnerable Design Elements:
Most upland LID practices have several common design elements, each
of which carry different
vulnerabilities under changing climate conditions.
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Inlet Erosion and Bypass:
Runoff enters the BMP through an inlet, which may be a curb cut or
sheet flow. As the intensity of storm
events increases and runoff volume is concentrated into fewer,
large events, the risk of inlet erosion
increases. Often, inlet erosion is the result of an existing design
flaw, which may be exacerbated by
climate change. Common causes of inlet erosion are:
• Improper design elevations
• An undersized curb cut
• Insufficient bypass measures for storms larger than the design
storm
Severe channelization and scour may undermine structural components
of the facility, leading to failure.
Inlet erosion can also contribute to performance failures by
causing cascading impacts throughout the
rest of the facility by depositing additional sediment in the bed,
or short-circuiting treatment.
To help avoid erosion, some “offline” practices – practices outside
the normal runoff flow path – may
use bypass devices that prevent excess runoff from entering the
facility at all. This can be controlled
using flow splitters or setting an inlet at the maximum ponding
height to moderate how much runoff
can enter the facility. However, increasing storm intensity may
result in larger runoff volumes bypassing
treatment in order to protect the structural integrity of the
BMP.
Clogged filter media:
As the runoff moves through the facility for treatment, any
additional sediment from erosion in the
contributing drainage area, or from the damaged inlet can be
deposited in the filter bed. The filter bed
media is a specialized sandy mixture designed to promote
infiltration. Significant sediment deposition in
the filter bed can clog up the media, reducing the permeability of
the filter media, and subsequently, the
total runoff reduction capacity of the practice.
Vegetation:
While not part of all upland LID practices, vegetation is a
critical component of many BMPs, such as
bioretention and green roofs. Vegetation is already one of the most
challenging maintenance needs for
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bioretention practices. As temperature increases and the
precipitation events become more intense, but
also more infrequent, studies have shown that vegetation in LID
practices will suffer from drought stress
in summer due to increasing evapotranspiration (Stovin et al.,
2013; Vanuytrecht et al., 2014). That said,
if an appropriate plant community is selected and properly
maintained, increasing evapotranspiration
can also help bioretention practices retain/reduce more stormwater,
resulting in a larger volume of
runoff reduction in summer. Thus, these same studies have
accentuated the need for a deep substrate
and a mixture of vegetation with diverse stress levels to balance a
trade-off between runoff reduction
and drought risks in the future (Sohn et al., 2019).
Outlet Erosion and Overflow:
Outlet structure design often depends on the design of the
facility. “Online” practices – practices placed
within the normal runoff flow path – typically have overflow
structures. Some designs have a
downgradient opening such as a gravel channel or curb cut that
allow excess flow to exit the system and
proceed along the normal stormwater runoff flow path. These designs
can be prone to erosion if the
overflow velocities are enough to mobilize undersized cobble in the
overflow channel or create a
“firehose” effect through the curb cut.
Other outlet designs use riser structures that accept water that
exceeds the maximum ponding depth
and conveys it to the underground storm drain pipes.
Either design means that more intense storm events are likely to
result in more overflow. Risk of
overflow is much higher for online practices than offline practices
because they fall within the regular
runoff flow path and therefore are more likely to be subjected to
more flow during intense events.
Tidal Impacts:
While not a vulnerability for a particular zone of a typical LID
practice, it is also important to note the
potential structural and performance failures that can be caused by
high groundwater tables and tidal
flooding as sea levels rise. Infiltration-based LID practices can
be rendered completely ineffective
without significant design adaptations if implemented in low-lying
regions of the Chesapeake Bay
watershed. A rising groundwater table reduces the capacity of
infiltration BMPs by filling otherwise
available pore spaces. This, along with potentially submerged
outlet structures during tidal flooding
events, can cause back-up and permanent ponding that risks
increasing peak flows with the installation
of infiltration-based LIDs (Joyce et al., 2017).
Rising sea levels are also leading to saltwater intrusion in
coastal communities. While usually more of a
concern for drinking water protection and agricultural production,
saltwater intrusion can also impact
the performance of stormwater management practices. High chloride
levels in BMPs can impact survival
of vegetation, as evidenced in many roadside bioretentions, and
there is some evidence that the
presence of large amounts of salt in stormwater BMPs may reduce
their effectiveness for total
phosphorus removal (Soberg et al., 2020).
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Figure 4. Examples of vulnerable design elements in upland LID
practices.
Inlet Erosion Bed Clogging Distressed Vegetation
Pollutant Removal Performance Impacts
Past experiments have consistently shown that LID systems perform
better with low-intensity and short-
duration storms under dry conditions (Alfredo et al., 2010; Gülbaz
and Kazezylmaz-Alhan, 2017). As
climate change increases the intensity of storm events, it is
important to understand the potential
overall impacts to runoff reduction and pollutant removal
performance.
The impact of climate change on upland LID practices is limited to
a handful of field studies and a few
additional modeling studies (Table 1). So far, there have been
mixed results with regard to overall
impact, though much of the difference may be related to the ability
to calculate overflow vs. bypass in
online and offline facilities.
Table 1. Summary of Select Climate Change Pollutant Removal Impact
Studies
Citation Type of Study BMP Performance Metric
Change in Performance
Catalano de Souza et al., 2016
Field (extreme weather as proxy for climate change)
Bioretention (offline)
Bypass volume 40% bypass during extreme events vs 23% bypass during
non-extreme events
Butcher, 2020 Modeled Bioretention Overflow volume 11% increase in
overflow volume by 2055
Alamdari et al., 2020
U.S. EPA, 2018 Modeled Mixed BMP removal efficiency
0-10% decline (TSS) 0-6% decline (TN) 0-5% decline (TP)
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Overflow/Bypass
Hathaway et al (2014) found that the volume of overflow from a
series of bioretention systems in North
Carolina substantially increased by 70-136% under climate change
scenarios. This is of particular
concern because it represents uncontrolled, untreated runoff from
the contributing watersheds.
In another study, a bioretention in New York was evaluated under
both normal and extreme
precipitation conditions. The site rarely ponded, and overflowed
only once (during Hurricane Irene),
generating an insignificant volume of overflow (Catalano de Souza
et al., 2016). However, the
researchers found that the site’s performance was more often
hindered by inlet bypass. Specifically, the
facility captured only 60% of runoff generated in its drainage area
during extreme events, compared to
77% of runoff generated during non-extreme events.
Both studies concluded that surface storage volume and infiltration
rate appeared to be important in
determining a system’s ability to cope with increased yearly
rainfall and higher rainfall magnitudes. They
also found that the total amount of precipitation and peak-hourly
intensity increase are significant
predictors of, and negatively correlated with, the facility’s
stormwater capture performance (Catalano
de Souza et al., 2016).
The 90th Percentile Storm
Another way to approach quantifying anticipated changes to BMP
pollutant removal performance is by
looking at projected changes in the 90th percentile storm event,
commonly used as the basis for the
water quality design storm across the Chesapeake Bay Watershed. A
preliminary analysis by CSN of
storm size distributions in the Washington D.C. metropolitan area
shows there has been little movement
in the 90th percentile storm event in the past 10-15 years (details
in Appendix A). Comparing rainfall
frequency tables compiled for Reagan National Airport for 1990-2020
vs 1977-2007, the 90th percentile
storm increased by 6% from 1.14” to 1.21”. In other words,
designing a BMP to capture the 1” storm
event now treats 85% of annual rainfall, rather than 88%.
Looking forward, a more detailed analysis by Butcher (2020) found
that the predicted intensity of the
90th percentile, 24-hour event does not show a consistent increase
at most Maryland stations under
future conditions, suggesting that many water quality BMPs are
likely to continue to provide expected
services under future climate. However, additional modeling
analysis found that while bioretention
facilities can be expected to capture and treat the future 90th
percentile storm without significant
overflow, there is an increase in median overflow volume across the
full range of storm events of 11%
by 2055 and 21% by 2085 (Butcher, 2020).
Pollutant Removal Efficiencies:
Other studies have looked beyond overflow, attempting to model
changes in pollutant removal
efficiencies – change in pollutant loads from the inlet to the
outlet of the practice – under different
climate change scenarios. The findings generally revealed that the
combination of larger storms, higher
intensity, longer duration, and wetter initial conditions
diminished the benefits of LID systems (Fassman
and Blackbourn, 2010; Hood et al., 2007; Jackisch and Weiler, 2017,
Wang et al., 2016). However, it
remains unclear as to which storm factors contribute more to LID
performance than others, and the loss
in pollutant removal performance has rarely been quantified.
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One recent modeling analysis using projected climate change
conditions found that median BMP
removal efficiency for TSS, TN, and TP were projected to decline by
6%, 7%, and 11%, respectively,
under a moderate future emissions scenario (RCP4.5); and 11%, 12%,
and 17%, respectively, for a high
emissions scenario (RCP 8.5) (Alamdari et al., 2020). Similar
results were found in modeling analysis
conducted by the U.S. EPA (2018) with TSS, TN and TP efficiencies
declining between 0-10%, 0-6% and 0-
5% respectively. Other studies have found that pollutant removal
efficiencies by LID practices were
largely unaffected by climate change in modeled scenarios because
the amount of pollutants present in
the urban area was constant even when the rainfall volume increased
(Baeka et al., 2020).
Researchers have also investigated the potential effects of
seasonality on pollutant removal
performance of LID systems, but the results were mixed. Some
empirical studies have found better LID
performance in summer than winter (Emerson and Traver, 2008;
Lewellyn et al., 2016) because
increasing temperature reduces water viscosity and thus increases
hydraulic conductivity of the soil
medium. However, the dependency on temperature varied by the type
of LID system and depth of soil
medium. Further, a contradicting study found fewer seasonal
variations in LID performance compared to
conventional BMPs (Roseen et al., 2009).
The Conveyance Practices
SUMMARY OF VULNERABILITIES
Conveyance practices carry significant risk because of their
importance and proximity to transportation corridors. Water quality
is less of a focus for this range of practices, so declining
pollutant removal performance is not well documented.
Conveyance practices are found in older developments, built prior
to current detention standards. The age of these systems and
likelihood that pipes and channels were undersized, increases the
risk of failure and flooding.
The transition from upland areas to the conveyance network also
represents a potential chokepoint within the urban drainage system.
Backups above the storm drain pipe system can create unintended
detention in the urban landscape that provides an additional,
inadvertent amount of volume control to manage during large storm
events.
Key vulnerabilities exist at inlets, outlets and from loss of
capacity. Inlets and outlets are at increased risk of erosion from
high flow events. Loss of capacity can occur due to high levels of
sedimentation within the conveyance practice, or due to tidal
flooding.
The BMPs:
• Vegetated Swales
• Open Channels
Conveyance practices serve a critical, and more traditional,
function in stormwater management:
moving water safely from urban areas to their receiving streams on
both private property and public
rights-of-way. These practices often run parallel to, or beneath,
transportation corridors and close to
houses and businesses. As a result, these practices inherently
carry more risk of catastrophic failure and
are often the highest priority for local governments. With the
exception of step-pool systems, there has
yet to be much implementation of water quality-focused retrofits in
this zone of the urban drainage
network, so there is less focus on pollutant removal failure or
diminished performance. However,
structural and catastrophic failure among these practices are among
the greatest concerns for
stormwater managers because of the impacts on public health and
safety.
Further, the transition from upland areas to the conveyance network
also represents a potential
chokepoint within the urban drainage system. Backups above the
storm drain pipe system can create
unintended detention in the urban landscape that provides an
additional, inadvertent amount of
quantity control to deal with during large storm events.
Deteriorating streets and parking lots, and
clogged storm drains also increase the initial abstraction and
event runoff storage found in aging urban
subwatersheds. Paradoxically, most of the increased future flood
damage will occur outside of and up-
watershed from the defined floodways and floodplain insurance zones
that have been the major focus
of past floodplain management efforts. Floodwaters will back up
above the 10-year storm drain
“chokepoint” where the “softer” drainage path and LID practices can
be overwhelmed by runoff volume
and corresponding erosive velocities.
Vulnerable Design Elements to Catastrophic and Structural
Failure
Vulnerability of practices in this zone is different than LID
practices because their primary function is
most often conveyance, not pollutant reduction. There are three
primary vulnerabilities in conveyance
practices:
• damage and erosion at the inlet;
• damage at the outfall into the headwater transition zone.
Ditch retrofits, bioswales, and dry-channel regenerative stormwater
conveyances (RSCs) do perform
pollutant reduction functions and therefore should be evaluated for
vulnerabilities as both conveyance
and LID practices.
Inlet and Outlet Erosion:
One similarity of conveyance practices to the upland LID practices
is the vulnerability at the inlet and
outlet. These conveyance systems are necessarily “on-line” and
collect runoff from much larger
contributing drainage areas than the highly dispersed LID
practices, and therefore are subject to
significant runoff volumes. Ditch and open channel designs with
steep slopes or insufficient pre-
treatment may experience similar scour and gully formation that can
undermine the integrity of the
channel. While engineered conveyance systems are designed to be
non-erosive for the 2-year storm
event, some of the “softer” BMPs, including grass channels and
swales may not be non-erosive for larger
storm flows. Further, non-engineered systems, like some rural
roadside ditches or overflow from small
LID practices may be more vulnerable.
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These conveyance systems either convey flow to a larger detention
practice, as will be discussed in the
next section, or to the headwater transition zone. The headwater
transition zone is the slope or channel
that extends from an upland runoff source to the stream network.
This zone has an exceptionally high
potential for sediment erosion due in large part to a combination
of uncontrolled stormwater runoff
from upstream development, inadequate energy dissipation structures
below the outfall and extreme
storm events that exceed the design capacity of the channel (Group
2, 2019).
Loss of Capacity:
Within the conveyance channel, there are three primary ways of
losing capacity within the system:
• significant sedimentation within the ditch, channel, or storm
drain pipe.
• rising groundwater tables or submerged outfalls due to tidal
flooding
• submerged outfalls due to backwater from undersized downstream
pipes
More intense rainfall, and failure or bypass of LIDs can cause more
erosion or sediment transport in
upland areas of the watershed that will increase the likelihood
that more sediment will be deposited
within the conveyance network. Non-engineered roadside ditches may
also experience erosional and
depositional processes similar to ephemeral streams under frequent,
high intensity events, leading to
sediment build-up in sections of the ditch system. As sediment
builds up, there is less capacity within
the conveyance system to transport runoff within the channel or
pipe, potentially leading to backups or
overflow.
Figure 5. Examples of vulnerable design elements for conveyance
practices.
Inlet Erosion Outfall Erosion Loss of Capacity
Submersion of outfalls may mean that runoff has no place to go at
the end of the channel or pipe, and
water will back up into the upland areas. This could be due to
undersized pipes downstream that are
unable to handle high intensity storm events, or an outfall that is
submerged by tidal flooding.
Alternatively, rising water tables may mean that ditches or aging
storm drain pipes may fill will
groundwater. As runoff competes for space in these conveyance
systems with groundwater, tidal
floodwater, or backwater, failure is more likely. This type of
capacity loss is becoming increasingly
15
common in coastal communities that are seeing the number of days
with tidal (“blue sky”) flooding
steadily rise.
Ponds and Wetlands
SUMMARY OF VULNERABILITIES
Stormwater ponds, particularly older, “legacy” ponds carry the
greatest risk of any stormwater practice because of their age and
their location within the watershed.
Inlets, forebays, risers, emergency spillways, and outfall channels
are all vulnerabilities of legacy ponds under climate change. There
are also concerns about the capacity of stormwater ponds under
future hydrologic conditions.
Loss of capacity, leaching, and resuspension of pollutants are the
primary mechanisms affecting future pollutant removal performance
under climate change conditions, though the anticipated changes in
pollutant removal performance have not been well-quantified.
The BMPs:
• “Legacy” Ponds
• Wet Ponds
• Stormwater Wetlands
Stormwater ponds represent the greatest potential risk for
catastrophic failure within the urban
drainage network because of their age, and their position within
the landscape. Stormwater ponds
collect runoff from large contributing drainage areas, often
anywhere from 25 to 400 acres in size and
are typically designed to safely pass the 100-year, 24-hour storm
event. For the past several decades
they have been the water quantity control practice of choice across
the Chesapeake Bay watershed. But
according to the 2017 infrastructure report card, more than 15,000
dams in the United States are listed
as high risk due to the potential losses that may result if they
failed, and yet by 2025, seven out of 10
dams in the United States will be over 50 years old (ASCE, 2017).
While that report focuses primarily on
larger, regulated dams, the small dams associated with stormwater
ponds are also aging and likely to be
in a similar maintenance condition.
Failure in this zone is not limited to catastrophic failure, as the
impacts can range from loss of water
quality performance, because of natural aging and resuspension, to
structural damages. Further, due to
climate change, in most future scenarios there is an increase in
both the social and economic risks in
comparison to the present risk level failure (ASCE, 2017;
Fluixa-Sanmartin et al., 2016; Mallakpour et al.,
2019). In many cases, a failure can begin without being
catastrophic, but can escalate if not addressed.
Vulnerable Design Elements:
Just as stormwater ponds have higher risk levels than other zones,
they also have more vulnerable
design elements. Inlets, forebays, risers, emergency spillways, and
outfall channels are all vulnerabilities
16
of legacy ponds under climate change. There are also concerns about
the capacity of stormwater ponds
under future hydrologic conditions (FEMA, 2018).
Figure 6. Examples of vulnerable design elements for stormwater
ponds.
Erosion at the Inlet Loss of Capacity – Full Pilot Channel
Embankment Failure Damage to Emergency Spillway
Age and Capacity Loss:
The majority of stormwater ponds were constructed in the previous
century with limited observation
data and with flood hazard assessments based on the natural water
regime at the time (Ho et al., 2017).
Therefore, their construction was based on outdated, historic
precipitation data and likely did not
incorporate the current and possible future changes in the
hydrological condition. There is some natural
“insurance” built into a number of pond designs as engineering
criteria for overflow, bypass, freeboard
and spillways tend to be conservative, and include factors of
safety, which may offset some risk from
future changes in hydrology.
In addition to outdated designs, capacity will be further limited
as the conveyance network emptying
into ponds deposits sediments, and the build-up over the productive
years could decrease the water
storage volume. This is largely an issue of deferred maintenance as
a result of communities and
homeowner associations lacking the financial and technical
resources to dredge and restore pond
17
capacity. While not a direct impact of climate change, the current
maintenance condition of these ponds
will be exacerbated by future climate change, as greater
precipitation intensity and subsequent surface
runoff, increases the expected flooding and overtopping events (Lee
and You, 2013; Shoghli et al., 2016).
In addition, if the water carries more and bigger suspended
material (including trees, branches, or
debris) this could lead to blockages and further loss of capacity
(Paxson et al., 2016).
Inlets, Outlets, and Spillways:
As more intense rainfall, coupled with changing land use
conditions, leads to more soil erosion, there
are a number or potential structural and performance failure
mechanisms within the stormwater pond
system due to increased sedimentation in the ponds (Salas and Shin,
1999; Yang et al., 2003).
Increased sedimentation can worsen the abrasion and erosion
processes on any mechanical equipment,
inlets, or the spillways (British Columbia et al., 1998), thus
compromising their reliability. Similar to on-
line conveyance practices, many ponds also have pilot channels with
steep slopes or insufficient pre-
treatment that may experience similar scour and gully formation
that can undermine the integrity of the
channel.
Diminishing Performance Impacts
Stormwater ponds, traditionally the water quantity practice of
choice, have also recently become a
target for retrofitting to achieve enhanced water quality gains.
That means there is also a concern about
impacts on pollutant removal performance under climate change. Most
of those concerns are related to
potential leaching or resuspension of pollutants due to high flow
events and increasing temperatures.
Sedimentation is the primary removal mechanism in wet detention
ponds for several stormwater
pollutants, though other processes like adsorption, microbial
degradation, and volatilization can also be
important (Scholes et al. 2008). Removal of suspended solids (TSS)
and other pollutants associated with
solids via sedimentation depends mainly on long hydraulic residence
times (HRT) leading to higher
removal rates (U.S. EPA 1999; Vollertsen et al. 2007). High flows
may disrupt the settling process and
shorten the HRT of stormwater retention ponds during extreme
conditions, which may lead to higher
pollution concentrations and loads being released from the ponds
(Sharma et al., 2016).
Other studies using model simulations have shown that the climate
change increase of rainfall intensity
led to an increase in the pollutant concentrations discharged from
the catchment. The higher flows,
combined with the increased inlet load caused a decrease in the
pond removal performance (Morgan et
al., 2007; Sharma et al., 2016). One study found that while the
median pollutant removal efficiency
remained relatively constant under the climate change scenario
(32.5% vs 33.9% removal), the median
yearly TSS load increased by 14% (Sharma et al., 2016). Another
study found that simulations of more
frequent storm events did not impact pollutant removal performance,
but that increasing the
magnitude of the storm increased the likelihood for loss of
performance, with a decrease of 20% from
the median removal efficiency when the storm size was increased
from 1” to 5” (Morgan et al, 2007).
There is evidence that one potential cause for this loss in
performance could be due to sediment-bound
pollutants that can be resuspended back into the water column by
high velocities during storms,
particularly in shallower ponds and wetlands (Davies and Bavor,
2000).
18
Another issue expected to become more common is the release of
pollutants, particularly P and TSS
from ponds due to leaching and resuspension. Periodic and sometimes
persistent thermal stratification
has been observed during summer months in stormwater ponds, even in
shallow ponds less than 6 feet
in depth (Song et al., 2013). Thermal stratification can be
especially problematic in warmer climates. This
thermal stratification can create persistent anoxic conditions that
lead to the release of phosphorus
from pond sediments, reducing removal efficiency and even becoming
a net source to receiving waters
(Erickson et al., 2018).
Stream Corridors and Shorelines
SUMMARY OF VULNERABILITIES
While stream restoration and shoreline management practices are
designed, in part, to protect against high flow events, their
position within the landscape makes them particularly vulnerable to
climate change.
Without new upstream runoff controls, urban streams and their
remaining corridors are likely to become more unstable, incised and
disconnected in the coming years, leading to loss in habitat
quality in the coming decades. While new stream and floodplain
restoration techniques are promising, the need for designs that can
withstand extreme flow conditions may detract from their habitat
and ecosystem service functions.
The impact on pollutant removal performance of these practices is
more complicated than the previous practices. Increasing stream
flow and erosion rates would theoretically increase the potential
pollutant removal rates from well- designed practices. However,
climate change may also impact assumptions about post-restoration
erosion rates for these types of BMPs.
The BMPs:
• Stream Restoration • Shoreline Management
Practices within the stream corridor are unique in the urban BMP
landscape because until recently,
stream restoration practices have not been considered a municipal
asset that needs to be managed or
maintained over the long run. This is changing as many of the
larger Phase I MS4 communities are now
creating methods to track and manage this new form of
infrastructure within the context of an asset
management system. While their location at the bottom of the
watershed makes them vulnerable to
impacts from increased precipitation, the understanding of the
specific risks is still evolving.
Flooding of stream corridors beyond the limits of the 100-year
floodplain certainly could be considered
catastrophic failure due to the risk to property and public safety.
Stream restoration can reduce these
risks, though are still susceptible to excessive flood events that
could destroy structural components of
the restoration, reduce pollutant removal performance and
potentially present public safety risks.
Shoreline management practices, while not technically part of the
stream corridor, deal with similar,
albeit tidal forces. Sea-level rise is a significant problem for
shoreline management practices, and this
19
risk was not factored into the pollution reduction credit, though
sea level rise considerations were
offered in the expert panel report’s appendix (Shoreline EPR,
2015). In addition, there is no standard
verification protocol for shoreline management practices that lose
function or are destroyed by higher
water levels.
Stream Restoration:
Stream restoration projects are designed based on a range of flood
events in the channel and floodplain,
and are intended to be sustainable, and allow for some natural
adjustments over time. Still, some
structural elements in the channel, especially the toe of the bank
and stream bed, are particularly
susceptible to damage. Certain fixed stream channel geometry, such
as outside of meander beds, are
also prone to damage.
Figure 7. Examples of vulnerable design elements for stream
restoration practices.
Structural Flanking Bank Erosion Headcuts
Some studies have shown approximately 20-30% of structural stream
restoration techniques experience
some degree of damage or loss of functionality (Brown, 2000; Dave
and Mittelstet, 2017; Miller and
Kochel, 2010). In general, these failures are driven by inaccurate
predictions regarding design
parameters (width, depth, meander radii, etc.) or because reference
sites are located in catchments
with less impervious cover than the restoration target reach
(Brown, 2000; Bernhardt and Palmer,
2007). These vulnerabilities are not specific to climate change,
but are likely to be exacerbated by
increasing flows and flashier storm events predicted due to climate
change.
Many assessments of structural failure among stream restoration
projects were conducted a decade
ago, and there have been numerous advances in the design approaches
since then. While constrained
urban environments still may require some “hard” design elements,
there has been a shift in the
Chesapeake Bay watershed towards “softer” bank protection practices
and more frequent reconnection
with the floodplain. Floodplain restoration projects may actually
provide more floodplain storage to help
manage extreme flood event caused by future climate. However, if
designers are given new
precipitation intensity-duration-frequency (IDF) curves, there is a
risk they will respond by making their
projects harder to withstand future design velocities, which may
reverse efforts to limit bank armoring
and promote more channel habitat, as outlined in recent guidance
from stream restoration experts
(Group 3, 2020).
20
Another advancement in the practice of stream restoration is that
there are now defined visual
indicators to help identify and correct structural and performance
loss (Group 1, 2019). Erosion of the
banks, headcuts, or flanking of structures can all be signs of
failure.
Shoreline Management:
Shoreline management practices are similarly difficult to assess.
While “softer” approaches to shoreline
management, such as living shorelines, are designed to adapt to
rising sea levels to some extent by
accreting sediments at roughly the same pace as sea level rise
(Davis et al., 2015), a number of harder
approaches are not as adaptable. Even living shoreline practices
are often constrained by their upland
environments, preventing them from migrating landwards to keep up
with the changing sea levels. For
living shoreline management projects, active marsh and/or wetland
intervention – including practices
like establishing landward buffers to allow inland migration over
time – may be needed to combat the
effects of sea level rise over time.
While only an eligible Chesapeake Bay BMP under very limited
conditions, the vulnerability of harder
shoreline practices is largely based on the age of the practice.
Because shoreline management practices
that include bulkheads or revetments are specifically designed to
protect against intense wave action
and sea level rise, recently designed projects should hold up well
during their design life. However, older
practices that may have used outdated climate projections or failed
to account for sea level rise at all
are likely to be undersized and prone to failure. Prolonged periods
of heavy rain or the crashing of storm
waves over a seawall or bulkhead can trap water behind them. If the
seawall can’t withstand the
pressure, it fails. Severe storm events can also erode the toe of
the wall, reducing its stability and
eventually leading to failure.
Pollutant Removal Performance Impacts
erosion and pollutant transport downstream (Bledsoe and Watson,
2001). Higher flows during
increasingly earlier snow melt could also result in earlier erosion
and incision of banks, making areas
more susceptible to spring precipitation events (Stryker et al.,
2018). Both higher temperatures and
longer storm durations would contribute to wetter conditions in the
watershed, producing antecedent
conditions where soils are more saturated and the watershed is more
vulnerable to any precipitation
event, even if not extreme in nature (Stryker et al., 2018).
The pollutant reductions calculated for prevented sediment are
typically based on pre-restoration
erosion rates. If those erosion rates are monitored, it could be
expected that the calculated
performance of stream restoration practices will increase in the
future as pre-restoration erosion rates
increase. Practitioners who rely on bank erosion rate curves, which
are more static, may eventually end
up with a more conservative estimate of pre-restoration bank
erosion rates. Therefore, climate change
further underscores the importance of pre- and post-restoration
monitoring to determining an accurate
estimate of the prevented sediment performance of stream
restoration projects.
21
influenced by changing hydrologic conditions. Although the annual
minimum streamflows have
increased during the last century, late-summer warming could lead
to decreases in the minimum
streamflows in the late summer and early fall by mid-century
(Demaria et al 2016). This influences
hyporheic denitrification because there will be potentially more
hyporheic exchange during baseflow,
but the seasonality may discount that benefit. Also, as more annual
flow accesses the floodplain, it is
likely that pollutant removal from floodplain reconnection will
increase.
Despite the potential for initial gains in pollutant removal
performance, the more important takeaway is
the high level of structural vulnerability of stream restoration
practices. There have been few, if any,
formal studies of climate change impacts on stream restoration
pollutant removal performance. While
theoretically there is potential for increased load reductions from
prevented sediment and floodplain
restoration practices, it is also very possible that those gains
can also be offset by a single extreme storm
event.
Shoreline Management:
Many of the same principles applied to stream restoration can also
be applied to shoreline
management. Projected sea level rise and anticipated increases in
large coastal storms are likely to
increase coastal erosion rates in the Mid-Atlantic (CBP, 2005).
That dynamic increases the prevented
erosion potential from well-designed shoreline management projects,
but it is unclear how long-lasting
those effects will be. It is likely that further work will be
needed to assess changes to practice lifespans,
as well as the effectiveness of softer shoreline management
techniques under extreme conditions and in
constrained environments without a landward buffer component to the
design.
Other Chesapeake Bay Restoration Approaches
SUMMARY
Non-traditional stormwater practices may also be vulnerable to
climate change impacts, but not in the same ways as practices
designed explicitly to capture, treat and convey stormwater.
Tree BMPs, especially those within the riparian corridor, are at
risk of changing temperature and moisture regimes that may shift
their successful ranges and impact mortality rates.
Impacts of climate change on programmatic BMPs, such as street
sweeping or urban nutrient management are poorly understood, but
likely to be minimal due to the current BMP crediting
protocols.
The BMPs:
• Riparian Buffers
22
The remaining restoration practices don’t fit as cleanly into the
previous urban drainage network
categories. Some, like the tree planting BMPs, can occur almost
anywhere throughout the landscape
Others, like street sweeping, are programmatic approaches to
pollutant reduction and therefore have a
very different risk framework.
Tree BMPs
There are three categories of tree planting BMPs within the
Chesapeake Bay TMDL framework: riparian
buffers, tree canopy expansion, and forest planting. Because tree
BMPs are relatively new to the urban
stormwater treatment world, there is little research to directly
assess climate change impacts on their
pollutant reduction capabilities since there was little baseline
performance data to compare to.
However, they still have vulnerabilities that can impact planting
success and mortality, which should be
considered part of their performance as a BMP.
Riparian buffers may be the most vulnerable tree practice because
of the expanding urban floodplain. As
the runoff volume and intensity increases, floodplain water
elevations rise, changing groundwater
conditions to a level that may be unsuitable for existing riparian
forest communities (Folzer et al., 2006;
Garssen et al., 2015). While specific riparian plant communities
may differ based on restoration design
objectives, changing conditions due to climate change may render
assumptions about groundwater
tables inaccurate in the coming years.
All tree BMPs will also likely have to plan for shifting tree
ranges due to increasing temperature and
precipitation patterns. More sensitive species have already shown
northward shifting ranges due to
temperature changes (Woodall et al., 2009), while others are
shifting westward to avoid the wetter
conditions on the east coast (Fei et al, 2017).
Similar to stream restoration and shoreline management, trees
traditionally do not perform quantity
control functions on a site scale, but also should not be
particularly vulnerable to increased storm
intensity. While the benefits of canopy interception for runoff
reduction are accepted and expressed as
detention provided on a regional scale, tree canopy practices are
not currently being used in the
Chesapeake Bay watershed to meet site-scale detention requirements.
More work is still being done to
better quantify the water quantity benefits of tree BMPs so they
can be further incorporated into future
stormwater design. Best practices for preserving existing canopy
and encouraging urban tree plantings
remain highly encouraged, but the changing climate’s impact on tree
BMPs will be unlikely to
significantly alter stormwater quantity objectives in the near
future.
Street Sweeping, Storm Drain Cleaning and Nutrient Discharges from
Gray Infrastructure (NDGI):
The Chesapeake Bay Program has also approved several “programmatic”
BMPs that can be used toward
achievement of the Chesapeake Bay TMDL. Vulnerabilities for these
non-structural practices should be
viewed differently because climate change may only threaten the
pollutant removal dynamics of these
practices.
Literature discussing the climate change impacts on these practices
is almost non-existent. The most
likely impacts to street sweeping and storm drain cleaning would be
associated with changing build-up
and wash-off dynamics. The anticipated shift towards more intense,
but more infrequent rainfall events
may improve the pollutant removal efficiency of street sweeping
programs that sweep less often,
though the impact is likely to be minimal. Likewise, higher
intensity events may mobilize more
23
sediments that could collect in storm drains, but there is not good
evidence to quantify the impact on
storm drain clean-outs. More analysis would be needed to support
any changes in presumed
efficiencies.
Finally, credit for eliminating NDGI and for urban nutrient
management are unlikely to be impacted by
climate change. The credit for NDGI explicitly excludes wet weather
overflows and discharges that are
more likely to increase under climate change scenarios (NDGI EP,
2014), while urban nutrient
management is driven by non-ag fertilizer application rates rather
than climate variables.
Moving to Resilient Design
SUMMARY
Resilient design for stormwater best management practices can be
grouped into two categories: sizing criteria, and resilient design
and maintenance adaptations. The series of resilient stormwater
design principles outlined in this section can help guide efforts
to update the next generation of design specifications.
In addition to protecting future projects from long term damage and
risk of failure, it is also worth evaluating whether new design
standards have the capability to not only protect against
performance loss due to climate change, but to enhance runoff
capture and pollutant removal rates over time.
The Chesapeake Bay Program has an opportunity to advance design and
resilient adaptation strategies for a wide range of restoration
practices, but further investigation will be needed to develop
recommendations that are actionable.
Having established the potential risks and vulnerabilities that
climate change exposes or exacerbates
among stormwater BMPs and infrastructure, it is important to shift
the focus to changes in design
criteria, BMP sizing and/or runoff modeling procedures that can
make the next generation of
stormwater practices more resilient to future rainfall and runoff
conditions. In addition to protecting
future projects from long term damage and risk of failure, it is
also worth evaluating whether any of
these changes have the capability to enhance runoff capture and
pollutant removal rates over time.
A series of foundational principles and more specific potential
design adaptations are provided below to
begin the movement toward more resilient stormwater design. The
Chesapeake Bay Program
partnership also has an opportunity to support further actions to
address resilience needs among other
restoration practices implemented across the watershed. A list of
recommended actions is offered as a
conclusion to this report.
Resilient Stormwater Design Principles
The following principles are proposed to guide the recommended
development of next generation
stormwater design standards and specifications:
1. Comprehensive Watershed Management – Design with consideration
for watershed context to
seek effective solutions for interrelated uplands, conveyance
systems, regulatory and non-
regulatory floodplains, and stream corridors. Working
collaboratively across agencies, seek
24
projects and designs that are resilient to both changing climate
and land use conditions in the
community.
2. Sizing – Design using sizing criteria that provides an
acceptable level of risk under future climate
conditions. There are multiple approaches to resilient sizing
criteria, that may include the use of
projected IDF curves, adding a “factor of safety” to historic
precipitation data, or establishing
over-management criterion for quantity and rate control.
3. “Flow-plains” – Design the conveyance and treatment system to
have capacity for safe overflow
when there is failure. This “flow-plain”, similar to a floodplain
for a stream or river, is an
adaptation of the cloudburst approach to extreme storm event
management. When overflow
exceeds the capacity of the system, manage the plane/slope/distance
to an area that is less
vulnerable to flood damage. For example, directing excess flow to a
buffer, forest or designated
ballfield to avoid impacts on housing or transportation
infrastructure.
4. Full-Cycle Implementation – Design with a “full-cycle” approach
to implementation. Establish
specific performance and maintenance targets for new
implementation, including an anticipated
design life, removal efficiencies for pollutants of concern,
relevant maintenance indicators, and
adaptive management for vegetation. Targets would ideally be tied
to assessment timelines and
triggers for management actions including repairs or
“makeovers”.
5. Redundancies – Design with redundancies in the system – both
within the practice and across
the site and conveyance system. On a site scale, “treatment trains”
route runoff through a series
of BMPs in succession, increasing the opportunity for capture,
infiltration, and pollutant removal
along the way. Within practices, redundancies can provide
protection against increased
maintenance burdens, such as secondary design elements that safely
pass excess flow if an
overflow structure clogs.
6. Performance Enhancers – Design using “performance enhancers” for
water quality to provide a
buffer against future increases in loads or reduced efficiencies.
Media amendments, “smart”
BMPs, and stronger vegetation guidelines may all provide improved
pollutant removal function
under both current and future conditions. Adapting new standards
for these BMP
enhancements may advance water quality goals or, at a minimum,
provide a buffer against any
decline in performance due to climate change.
Stormwater BMP Design Adaptations
Sizing
One option for improving practice resiliency is to revisit
stormwater design sizing criteria. There are
several ways that sizing can be approached. One option is to keep
the same design storm criteria, but
update the underlying precipitation data used to arrive at the
designated storm events. Due to the
expected increase in precipitation intensity (5-35% by
mid-century), using projected IDF curves would
result in an increase in BMP and infrastructure sizing (Wood,
2020c).
Another approach is to use a “factor of safety” to increase sizing
criteria. With the approach, rather than
use projected IDF curves directly, future precipitation analysis
was conducted and then a 20% factor of
safety was added to existing design criteria. This approach has
already been utilized by several
communities (Wood, 2020c). Less likely would be a change to the
actual design storm (ex. Increasing the
conveyance storm from 10-year to 15-year).
25
Over-management criteria are another opportunity to provide more
conservative sizing criteria. If a
community is not comfortable with the precision of future rainfall
projections, rather than try to predict
that the future 100-year storm intensity, they will set a criterion
that developers release the 150-year
post-development storm at the 100-year pre-development level to
provide a factor or safety.
The trade-offs for any of these options are cost and available
space. Increased storage volume means
larger, and therefore more expensive, practices. In constrained
environments, land use and other major
infrastructure restrict available space to simply increase the
storage volume. In these instances,
consideration must be given to other resilient design adaptations
that may provide similar benefits but
with a smaller physical and financial footprint.
Resilient Design Adaptations
“Smart” BMPs (Continuous Monitoring and Adaptive Control): Smart
BMPs have already been deployed
as a retrofit for legacy stormwater ponds in the Chesapeake Bay
watershed. These techniques integrate
information from field deployed sensors with real-time weather
forecast data (i.e., NOAA forecasts) to
directly monitor and make automated and predictive control
decisions to manage stormwater storage
and flows within the pond (Quigley and Lefkowitz, 2015). In other
words, prior to a predicted rainfall
event, the outlet of the pond can automatically draw down the water
levels, creating additional capacity
in anticipation of the storm. Monitoring of ponds with smart BMP
technology has shown potential for
improved water quality results and more downstream channel
protection compared to traditional
stormwater ponds (Braga et al., 2018; Marchese et al., 2018;
Muschalla et al., 2014;), though the impact
on larger design storms is less clear (Schmitt et al., 2020).
Media/vegetation amendments: There are opportunities to improve the
resilience of LID practices by
adjusting media to promote greater infiltration and soil storage
capacity, or adapting vegetation
guidelines to reflect changing growing conditions. Performance
enhancing devices (PEDs), such as
biochar, iron amendments, wastewater treatment residuals and
internal water storage (ISW), all have
demonstrated the initial ability to improve the runoff reduction
capacity and/or pollutant removal
efficiency of LID practices (Hirschman et al., 2017). Some of these
may help add resilience to the design,
or otherwise offset performance losses due to climate change.
Adapting vegetation guidelines has the potential to serve as both a
performance enhancer, as well as a
necessary maintenance adjustment to reflect changing growing
conditions. In tidally influenced areas,
rising groundwater levels may necessitate a shift to more
salt-tolerant plant regimes or to more wet-
tolerant species (Horseley Witten Group, 2015). A range of further
actions can improve the overall
performance of vegetation in LID practices, including:
• Use local natural plant communities as reference
landscapes.
• Provide dense cover of the BMP surface with layers of
vegetation.
• Intensely manage the plantings for the first 3 growing
seasons.
• Use an adaptive management approach for long-term O&M.
Treatment Trains: Stormwater treatment trains are already relied
upon in some new and redevelopment
stormwater projects. The concept is to route runoff through a
series of BMPs in succession, increasing
the opportunity for capture, infiltration, and pollutant removal
along the way. The combinations of
practices are variable, depending on site conditions, and provide
redundancies if one of the practices in
26
the chain were to fail or be bypassed (Horseley Witten Group,
2015). The treatment train approach can
provide enhanced pollutant removal performance for the site, and
reduce long-term maintenance costs
by for the entire chain by provided layered treatment (Doan and
Davis, 2016; Fassman and Liao, 2009;
Pronchik, 2016).
Inlet/Outlet Protections: Enhancements to the traditional BMP
“plumbing” is another route to ensuring
the BMPs can safely accept and pass high intensity rainfall events.
Proper sizing of inlet and outlet
structures, avoiding steep slopes that can raise the risk of
erosion, providing more effective pre-
treatment, and re-thinking overflow and outfall design are all on
the table. However, more work is
needed in this area to develop specific design alternatives.
Better Maintenance: An often-overlooked element of the BMP lifespan
is long-term maintenance. Many
BMPs are designed without consideration of how to maintain them –
either by designing practices that
are likely to require frequent maintenance because of a lack of
pre-treatment, or even failing to consider
access to the BMP for routine maintenance activities. As climate
change increases the need for both
routine and non-routine maintenance of stormwater practices, there
should be consideration of
mechanisms for rapid inspection and maintenance delivery.
Coastal Design: As sea level rise and rising groundwater threatens
to submerge outfalls or infiltrate
drainage pipes, solutions will be needed to maintain capacity in
the urban drainage network. There are
several approaches being explored, including:
• Increased pipe/ditch sizing to compensate for lost capacity
• Elevating outfalls above projected high tide levels,
• Installing check valves to prevent backups,
• Implementing pump systems to move water through the conveyance
network
Conclusions and Next Steps Based on the anticipated risks and
vulnerabilities to existing stormwater BMPs and conveyance
practices, this memo presents the case that it is now time to begin
development of the next generation
of stormwater design standards. By looking at a comprehensive
approach to resilient design – more than
just increased practice sizing – communities will have the
opportunity to not only mitigate the expected
impacts of climate change, but potentially improve the pollutant
removal performance of BMPs through
enhanced design and maintenance practices. Between lower
maintenance demands and increased cost-
effectiveness, the hope is that these future modifications will
provide cost-effective solutions over a
longer planning horizon.
While there is enough evidence to support a call to action, there
are a number of proposed steps that
the Chesapeake Bay Program partners may consider to move towards
implementation of new design
guidance:
27
Table 2. Summary of Possible Chesapeake Bay Program Strategies to
Promote Climate Resilience in
Urban BMPs.
Proposed CBP Strategy
Stormwater BMPs for New and Re- Development
Very High Given the number and complexity of stormwater BMP
designs, the main strategy should be to develop effective new state
stormwater design standards and specifications as updates to
existing state stormwater engineering design manuals. The effort
should look at new criteria to:
• Improve reliability in removing nutrients and other
pollutants
• Adjust BMP “plumbing” criteria to make sure they can convey
runoff from extreme storms without damage or loss of function
• Establish numeric triggers for BMP inspection and
maintenance
• Update BMP landscaping criteria to reflect species adapted to
future growing conditions.
• Promote smart BMPs that provide real time detention
An informal Bay-wide technical group could help facilitate sharing
of cost-effective BMP criteria that are resilient to climate change
and other factors that diminish pollutant removal performance over
time. Pending proposed changes to standard BMP design criteria,
revisit the Stormwater Performance Standards Expert Panel report to
make adjustments to BMP pollutant removal credits and qualifying
criteria.
Stream Restoration
High STAC-sponsored (or other funding mechanism) design charette
with stream restoration practitioners and researchers to establish
recommended best practices for stream, floodplain, and riparian
corridor design that is resilient to the range of extreme floods
expected under future climate scenarios. The Modeling Workgroup and
Watershed Technical Workgroup (WTWG) could coordinate efforts to
assess how extreme flooding might influence the nutrient and
sediment erosion, transport, and deposition from streambeds and
streambanks.
28
Shoreline Management
High Convene small group of shoreline management practitioners and
climate experts to recommend whether the existing BMP crediting
protocols (or qualifying conditions) need to be adjusted to assure
practice resiliency due to future sea level rise and blue-sky
flooding.
Stormwater Retrofits
Moderate Individual state stormwater/floodplain/dam safety agencies
should work together to develop stream-lined design criteria for
this class of projects.
Tree BMPs Moderate The Forestry Workgroup may wish to provide
guidance on which tree planting species and techniques will work
best in the warmer and wetter climate of the future. The FWG may
also outline research on both the risk of riparian tree mortality
during extreme floods, as well as impacts on canopy interception
and pollutant removal capabilities of tree BMPs.
Urban Nutrient Management, NDGI, and Street Sweeping
Low The effect of future rainfall patterns on air deposition,
erosion, pollutant accumulation and wash off functions are not well
known in urban watersheds (i.e., would these result in a meaningful
change in simulated nutrient and sediment export rates from
pervious and impervious land?) The Modeling Work Group or WTWG
could supervise a contractor to integrate future precipitation and
temperature scenarios into the hourly time step of pollutant export
simulations to an appropriate watershed model.
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